Ni-based alloys, in particular Ni—Cr—Mo—Nb alloys, are used in harsh environments that are highly corrosive because such alloys have superior corrosion resistance. In this way, these alloys are used in harsh environments in which there is the risk that Fe-based alloys will be corroded. Therefore, corrosion resistance at surfaces is particularly important.
In order to apply corrosion resistance of Ni—Cr—Mo—Nb alloy sufficiently, techniques concerning formation of passivation films are known (for example, see Japanese Unexamined Patent Application Publication No. 2015-183290). Since corrosion resistance is exhibited at the surface, and surface conditions are particularly important. If the surface is viewed microscopically, the surface is seen to be constructed of crystal grains. The surfaces of the crystal grains are sufficiently maintained by a dense passivation film. However, there is a problem in that the crystal grain boundary has less corrosion resistance. The reason is that in Ni—Cr—Mo—Nb alloys, deposits containing Cr or Mo may be formed at grain boundaries if conditions of heat treatment are not appropriate. Since the passivation film mainly containing Ni, Cr, Mo and O, which is effective for corrosion resistance, is difficult to be formed densely on the deposits, corrosion resistance may be deteriorated. Corrosion resistance may be further deteriorated by sensitization. That is, in a neighborhood of the deposits containing Cr or Mo, the Cr or Mo in the base material is dispersed to the deposits, and an absentee layer containing less of these elements is formed. Since Cr and Mo are effective for corrosion resistance, if the passivation film dissolves in a corrosive environment, corrosion occurs from this absentee layer of Cr and Mo, and thus, corrosion resistance is extremely deteriorated.
In view of the above object, a technique in which Ni-based alloy having no carbide is produced by performing solution heat treatment is disclosed (for example, see Japanese Unexamined Patent Application Publication No. Showa 57 (1982)-9861). Actually, according to this technique, the alloy has superior corrosion resistance at a step of shipment from the factory. However, since Ni-based alloy is used being processed as a pipeline, chemical plant, reaction vessel or the like, there may be a case in which the alloy is heat treated via these processings or weldings. In that case, if an inappropriate heat treatment is performed, there may be a case in which deposits containing Cr or Mo are formed at grain boundaries. Then, grain boundary corrosion resistance is deteriorated by the abovementioned mechanism, grain boundary corrosion is promoted, and in the worst case, a serious problem occurs to the extent that corrosion penetrates the material. In this way, it can be said to be a very important technique to prevent carbides containing Cr or Mo, which is effective element for corrosion resistance, from forming at grain boundaries.
A technique in which formation of carbides containing Cr and Mo is prevented in Ni—Cr—Mo—Nb alloy containing 11 to 20% of Mo is known (for example, see Japanese Unexamined Patent Application Publication No. Heisei 7 (1995)-11404). That is, this is a technique to prevent carbides containing Cr and Mo from forming by depositing NbC at grain boundaries by performing aging heat treatment at 600 to 800° C. for 1 to 200 hours. However, it requires aging heat treatment at 600 to 800° C. and a long time of 1 to 200 hours, and there is a problem in that it is not actually possible to perform the treatment after the pipeline, chemical plant, reaction vessel or the like is completed. That is, the technique is a method that is impossible to employ industrially. In addition, the publication describes nothing about size and density of NbC, and it is not clear whether or not NbC is stabilized by this technique.
A Ni-based alloy exhibiting superior grain boundary breaking resistance is proposed, which is developed by producing test pieces under conditions not depositing NbC, that is, solution heat treatment, and by evaluating with grain boundary corrosion resistance test while imparting stress (for example, see Japanese Unexamined Patent Application Publication No. Heisei 5 (1993)-255787). As mentioned above, in a condition in which carbides are in a solid solution, inappropriate heat treatment after assembling pipelines, chemical plants, reaction vessels or the like may cause formation of deposits containing Cr or Mo at grain boundaries, and thus, the technique is not practical.
Furthermore, a technique in which solution heat treatment is performed at 1000 to 1100° C. and rapid cooling is performed at not less than 200° C./sec so as to solid-solve carbide (for example, see Japanese Unexamined Patent Application Publication No. Heisei 5 (1993)-140707). Corrosion resistance can be reliably obtained if these conditions can be realized. However, it is not possible to actually perform the heat treatment and rapid cooling after the pipeline, chemical plant, reaction vessel or the like is completed, and thus, the technique is not practical.